Upgraded my son to a 20" hardtail - is there still a use for his 16" bike??

For skatepark you'd probably be OK, but might want a little taller riser bars and slick tires.

The Banshee had fairly tall bars so it should work well as a skatepark bike. If he looks too small on it (relative to how an adult looks on a freestyle BMX) I'd flip it and get a Cult Juvenile 16.

25t x 9t is pretty common for bmx gearing now - smaller cog in front to prevent hitting things, smaller cog in back to keep gear ratio relatively high. Unlike mtb, the priority is usually to going quickly over a short distance with higher torque. With the stock cassette ring in back on the banshee, gearing might wind up too low to reduce the chainring much.

The other difference in bmx geometry is a steeper heat tube angle.

For other options, the Fit Misfit was what we went with - quality is on par with the Cult and price is similar, but there are some small differences in geometry between them and IMO Fit has tended to offer cooler color options.

We've got the 16" Yoji around that my 49" 7yro still uses on the DJ lines etc. Its similar to your 16" bike. Its not a BMX bike, its a little rigid DJ bike! Works great and he goes hard on the thing. He has a 16" BMX bike that is is ready to use but still loves that little 16" Yogi at the moment for some reason. The 16" BMX is slighty, slightly big for him (United Recruit 16) but is likely the better fit right now, need to convince him to spend more time on it.